Explosive fracturing method

ABSTRACT

THIS INVENTION IS A METHOD FOR INCREASING THE PERMEABILITY OF UNDERGROUND FORMATIONS BY INJECTING A FLUID EXPLOSIVE INTO SUBSURFACE CREVICES AND DETONATING THE EXPLOSIVE IN THESE CREVICES. IN PARTICULAR IT RELATES TO THE PLACEMENT OF DETONATORS IN A PARTICULAR ADVANTAGEOUS WAY WHICH INCREASES SAFETY AND INSURES MORE CONSISTENT DETONATION.

Feb. 27, 1973 w. G. BEARDEN ETAL 3,718,088

EXPLOS IVE FACTURING METHOD Fllea April 23, 1971 2 Sheets-Sheet l WATER INVENTORS CLARENCE R. FAST BY WILLIAM G. BEARDEN HOM A. M LLINGER F|G.2 ATTORLNEL w. G. BEARDEN ET AL 3,718,088

2 Sheets-Sheet 2 2 I @m H LT I "III IIIII" III I ,I IIIIIHI II III' I EXPLOSIVE FACTURING METHOD IIII IIIIIIIIIIIIII Feb. 27, 1973 Filed April 23, 1971 IIIIIIII IIIIIIIII I I'I III INVENTORS CLARENCE R. FAST BY WILLIAM G. BEARDEN II I I I II I II IIII .I/IIIIIIIII I I IIII I I' I I II.

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III II I 'III I I I I I I I 'I'IIIIII I IIIII llllll I l I I I II I ll I I United States Patent 3,718,088 EXPLOSIVE FRACTURIN G METHOD William G. Bearden, deceased, by Edith Mae Bearden,

administratrix; Clarence R. Fast and Morton A. Mallinger, deceased, by Zella K. Mallinger, administratrix,

Tulsa, Okla., assignors to Amoco Production Company,

Tulsa, Okla.

Filed Apr. 23, 1971, Ser. No. 136,776 Int. Cl. E21!) 43/26 US. Cl. 102-21 7 Claims ABSTRACT OF THE DISCLOSURE This invention is a method for increasing the permeability of underground formations by injecting a fluid explosive into subsurface crevices and detonating the explosive in these crevices. In particular it relates to the placement of detonators in a particular advantageous way which increases safety and insures more consistent detonation.

CROSS REFERENCES TO RELATED APPLICATIONS Much of the prior art, and an improved liquid explosive fracturing medium is disclosed in the continuation-in-part of Clarence R. Fast, Ser. No. 95,005, filed Nov. 23, 1970, the original application being Ser. No. 3,511, filed Jan. 16, 1970, now abandoned.

A special method of placing a liquid explosive for such fracturing is disclosed in the US. application Ser. No. 30,751, filed for Clarence R. Fast, George C. Howard, and Riley F. Farris, on Apr. 22, 1970. In this same field but with somewhat less relevance is the application Ser. No. 875,843, filed Nov. 12, 1969, for Messrs. Clarence R. Fast, George C. Howard, and Morton A. Mallinger.

BACKGROUND OF THE INVENTION (1) Field of the invention For many years it has been known to be advantageous in treatment of oil and gas wells penetrating a low permeability formation to increase the effective permebility by isolating the pay zone, or a part of it, and applying increased pressure to a fluid confined in the zone until rock fracture occurs. This increases the drainage area exposed to the bore of the well and thus increases eflective permeability. More recently a number of investigators have proposed substitution of a fluid explosive for the hydraulic fluid, particularly if fractures exist in the formation, so that explosive can fill these cracks. This, of course, requires that the fluid explosive can propagate detonation within the narrow confines of the fracture.

We are aware that various liquid explosives have been suggested generally for the use we propose. For example, the Bureau of Mines has experimented with fracturing rock outcrops in which desensitized liquid nitroglycerin was displaced into the fracture system and detonated. Subsequent well data obtained with a downhole camera on this very shallow well (of the order of 625 feet) indicated localized casing damage in the perforated in "ice terval, and extensive fracturing. However, liquid nitroglycerin, even when desensitized, is not a material handled with ease by the customary service companies. Also, it has a viscosity at field temperatures not far above water and hence can leak away into the formations.

We are aware that Chestnut in US. Pat. 2,892,405 has disclosed and claimed hydraulic fracturing essentially equivalent to that which we wish to carry out, using sensitized nitromethane which is pumped into cracks and fissures adjacent a bore and detonated from the bore. This is a very much safer type explosive. However, the detonation arrangement proposed leaves much to be desired. Other workers have also issued various patents, but still without a good system for insuring explosive detonation. For a review of this prior art see US. patent application listed above in the cross references. This material is incorporated by reference herein. It has been taught in these references to use a suitable time bomb which is attached to the top plug which immediately follows the column of liquid explosive into the bore, or to drop such a detonator into the column, permitting it to fall through the liquid explosive mixture to the bottom of the well, or to place the detonator in the bottom of the well before pumping the liquid explosive into the bore. All of these arrangements have real disadvantages. It is certainly undesirable to place a detonator or time bomb in the Well until after all of the explosive has been pumped into the well, since there is always possibility of the bomb exploding prematurely. On the other hand, if the detonator is permitted to fall freely, or is attached to the upper wiper plug, it is possible that the detonator never reaches the bottom of the well. The further removed from the bottom of the well this detonator is at time of detonation, the greater is the possibility that it is not in contact with enough of the liquid explosive to insure that the main body of explosive is, in fact, ultimately detonated.

SUMMARY In this particular well treatment, a suitable mechanical arrangement, including what is called a plug catcher, is located an appropriate distance above the bottom of the well, but usually within the pay section (for example, about 50 to feet above the bottom of the pay section). The bottom wiper plug can pass through this plug catcher without difficulty, down through the tubing below the catcher and thus permit explosive to be injected through the bottom of the string. On the other hand, either the top wiper plug itself, or a blanking plug attached thereto, is caught by the catcher sub. This seals the bore. Below this upper wiper plug (or below the blanking plug if one is employed) a suitable time bomb is attached by a short length of flexible material, for example a rope, made up of material soluble in the explosive used, but not in water. Preferably this time bomb is attached at the bottom of a second time bomb which is attached in turn by a flexible connector, unaffected by any of the liquids ever present in the well during treatment, to the blanking plug, or to the top wiper plug (if there is no blanking plug). Thus when the top wiper plug is injected into the well following the charge of liquid explosive, ultimately the lowermost time bomb is detached from the wiper plug, or blanking plug, by the dissolving of the attaching rope. Ordinarily this will occur only after the bore has been sealed. The bottom time bomb thus falls to the bottom of the well where it is in the best location for detonation of the liquid explosive.

We find it desirable to use two time bombs in each treatment to minimize the possibilities of a misfire due to bomb malfunction. Also the condition of the blanked off part of the bore may be such that detonation can more easily occur from the upper rather than the lower time bomb.

After the top wiper plug has reached the lowest point possible in the well, the bore above the seal is preferably stemmed with several hundred feet of sand or a quick setting cement slurry or the like. This may be dumped, if no tubing is employed, in the region above the seal. If tubing has been used, the tubing preferably is detached at a point above the seal and pulled up suitably before using the stemming procedure.

BRIEF DESCRIPTION OF THE DRAWINGS The attached drawings form a part of the specification and are to be read in conjunction with it. In these figures the same reference numeral in different views refers to the same or a corresponding part.

FIG. 1 shows partly in cross section and partly in cutaway a diagrammatic view of the apparatus used in the well and at the surface for carrying out this invention, employing a string of tubing in the well.

FIG. 2 similarly illustrates the equipment employed in such a well in case no tubing is used in the well above the packer.

FIGS. 3 to 6 show in still more diagrammatic form various steps in carrying out the procedure of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 there is shown in cross section and cutaway a well penetrating a subsurface formation 11 above a pay zone 12 below which there is another formtion 13. The casing 14 of the well has been perforated in the pay zone and it is assumed that the formation is fractured either naturally or artificially, these fractures connecting with one or more of the perforations in the casing 14. The well itself is plugged back or bottomed at some point 15. In our process, liquid explosive is forced down through the well and into the fractures or fissures in the pay zone 12, this liquid explosive in the pay zone being subsequently detonated by detonation of the explosive in the bore.

In preparation for the introduction of the explosive, the tubing has been pulled from the well and a special assemblage of equipment has been lowered into the well. Starting from the bottom, there is a lower tubing 17, ordinarily called a tail pipe, which preferably is made of plastic rather than metal, and extends to a point near the bottom 15. Immediately above this is the plug catcher 18. This is simply a short section of tubing or a tubing coupling containing an annular inner projection 42 which, as the name indicates, ultimately catches or stops a plugging member, which is either the top wiper plug or a blanking plug mounted immediately below the top wiper plug. This plug catcher 18 is mounted on a drillable packer 19 which, after the tubing 16 has been run until the bottom end of the lower tubing or tail pipe 17 is close to the bottom of the well, is set in the usual way against the casing 14. Immediately above the packer is a short section called an anchor tubing seal assembly 20 which permits the tubing 16 to be detached from the drillable packer, for example by reverse rotation. Above this the tubing 16 extends to the wellhead. At the wellhead, we preferably provide a blowout preventer 21. Above this the tubing continues into a T 22 where a side string of pipe 23 is connected through an explosive-safe valve 24. A quick dis connect 25 is connected at the top of the tubing string which connects the tubing through a second explosivesafe valve 26, to a lubricator 27. At the top of the lubricator a union 28 (preferably nonhammer type) connects to the top member 29, which in turn connects through a valve 30 to a second pipe string 31.

After the packer 19 has been set and the equipment above the wellhead assembled, the pipe 23 is connected to the source of liquid explosive to be employed. The lubricator 27 is opened through union 28 to permit insertion of the top wiper plug assembly. This includes the top wiper plug 32. Immediately below it is sturdily mounted the blanking plug 33, which in turn is connected to the bomb connector 34. Attached (preferably flexible) to the connector 34 is the upper time bomb or detonator 35, the connection between it and connector 34 being insoluble in any of the liquids present. Below this in turn is the second time bomb or detonator 36 connected to the first time bomb 35 flexibly by rope 37, which is soluble in the liquid explosive.

In the case of use of a liquid explosive which is mostly nitromethane suitably sensitized by a minor amount of one or more additional chemicals (and this is discussed in the prior art already listed), simple experiments are carried out in advance of the well treatment to determine the time rate of dissolution of the rope 37 in the nitromethane. For example, we have found out that a -inch diameter rope of cellulose acetate will part under ordinary Well temperatures in the order of three-quarters to one hour. The diameter of rope chosen for a particular well treatment is such that after the lower time bomb 36 contacts the liquid explosive in the well, there is suflicient time for the top wiper plug and attached equipment to reach the plug catcher 18 before, or approximately at the time the rope 37 parts, so that the lower time bomb 36 will fall through the lower tubing 17 to the bottom 15 of the well. Of course, a thicker rope will dissolve slower. Poly- Vinyl chloride, cellulose nitrate, cellulose triacetate, and cellulose acetobutyrate are other examples of materials from which flexible ropes can be fabricated, which will dissolve slowly in nitromethane and hence are suitable for the flexible ether 37. A monofilament rope dissolves more slowly than one made out of twisted fibers.

The top wiper plug assembly is preferably surrounded while in the lubricator 27 by an inert fluid, for example, water, so that the flexible tether 37 does not commence dissolving until this top wiper plug assembly has passed into tubing 16 and is in contact with the liquid explosive.

The top member 29 of the lubricator is connected through valve 30 and second pipe string 31 to a pump and a source of an inert fluid, typically water.

The operation of the preferred method is shown in FIGS. 3 through 6. The drillable packer 19 (one current example of which is the Baker Oil Tools FAB retainer) is made up with a fill-up collar and plug catcher 18 and with a suflicient length of lower tubing or tail pipe 17 (preferably plastic) to extend from the final chosen position of the plug catcher to about roughly two feet above the bottom 15 of the well. This assembly is run preferably on a conductor cable, through a string of tubing can be used, and the packer 19 is set at the desired depth. A string of tubing 16 containing an anchor tubing seal assembly 20 is then run into the well. The seal assembly 20 permits latching the tubing 16 into the packer 19 and sealing this from the annulus 38, thereby affording a continuous conductor from the surface essentially to the bottom of the well. The upper end of this string of tubing is attached through T 22 to the valve 24 and pipe string 23.

A lower wiping plug 39 is then inserted in the tubing string at a position below the T 22, as shown by the dashed lines 40 in FIG. 3. The top of the tubing string above T 22 may then be sealed by closing valve 26, valve 24 opened (as shown in FIG. 3), and the liquid explosive 41 is injected into the tubing 16, forcing the lower wiping plug down the tubing. This plug 39 isolates the explosive from the fluids present in the lower portion of the tubing string.

The lower wiping plug has been chosen with sufiiciently flexible lips so that it will pass through the annular ring or stop 42 in the plug catcher 18 under ordinary pressure differentials.

After all the explosive is injected into the tubing, plus a little water or similar inert liquid simply to lower the top of the explosive liquid below the wellhead, lubricator 27 is attached to the top of the tubing string 16 by quick disconnect 25. The two time bombs 35 and 36 are then energized and set for the desired time before detonation, then attached as shown in FIG. 1 to the top wiping plug assembly. The assembly is mounted in the lubricator 27 with valve 26 closed. Top member 29 is then connected by union 28 to the lubricator 27. FIG. 3 shows the arrangement at the well at this point in time.

As shown in FIG. 4, valve 24 has been closed. Valve 30 is then opened, and an inert fluid, such as water, is pumped through line 31 to force the top wiping plug assembly down into the well. This in turn forces the liquid explosive 41 ahead of it, which in turn forces down the lower wiping plug 39, ultimately out the bottom of the lower tubing 17. The liquid explosive then is forced into the fissures or fractures 43.

We prefer to inject the liquid explosive 41 at a low rate, of the order of A, to /2 barrel per minute, and under low differential pressure. It is particularly important that the pressure on the column of liquid explosive be not suddenly increased, since some liquid explosives are sensitive to adiabatic compression and might prematurely explode.

As this process continues, ultimately the shoulder on blanking plug 33 strikes the projection 52 on the plug catcher 18 and the top wiping plug assembly is fixed in position. This seals off the tubing.

The soluble string or rope 37 has been exposed to the liquid explosive 41 during the entire trip through the tubing string 16 so that part of this rope has been dissolved. Preferably complete solution of the rope 37 does not occur until after the blanking plug 33 is seated in the plug catcher 18.

As an illustration in a practical embodiment, a cellulose acetate rope was prepared having a minimum dimension of the order of of an inch. This rope dissolved in nitromethane in approximately three-fourths of an hour. It should be pointed out that this Was a loosely braided rope made up of three strands of cellulose acetate fabric, approximately an inch wide before braiding, the threads of the fabric being of the order of to 7 of an inch in diameter. A similar rope which was coated with liquid beeswax before testing dissolved in approximately one hour and 45 minutes in nitromethane. The temperature at which these tests were conducted was about 75 F. Such a rope was employed in a test of this system in which it took approximately 30 minutes for the column of liquid explosive to pass through the tubing string until the blanking plug was in final position, which allowed about an additional one hour and 15 minutes for the rope (beeswax coated) to dissolve and permit the lower time bomb 36 to fall to the bottom of well 15, as shown in FIG. 5.

It is seen that this arrangement has located the two time bombs preferably employed, at advantageous positions for ultimate detonation. The top wiper plug 32 has wiped remaining liquid explosive from the tubing walls and accumulated this material ahead of the plug so that it is available both to dissolve the soluble rope for release of the bottom bomb and to produce a concentration of liquid explosive in the immediate vicinity of the upper time bomb 35. By the dissolving action on the rope 37, the lower time bomb 36 is finally released to rest in the bottom of the well where there is also accumulation of the liquid explosive 41.

Following the seating of the blanking plug 33, pumping ceases and the lubricator 27 is detached from the top of the tubing string 16. The tubing is raised, detaching it from the anchor tubing seal assembly 20. Preferably the well is then stemmed, i.e., the region above the packer 19 sealing the annulus and the top Wiper plug 32 sealing the tubing, is filled with a cement slurry (preferably quick-setting), sand, or some heavy inert material, such as fly ash in water slurry. We prefer to have a column of such material 45 extend upward above these seals for a distance of the order of 200 feet or so. The tubing 16 may then, if desired, be completely removed from the well. The arrangement during placing of such stemming material is shown in FIG. 6.

If two time bombs are used, they ordinarily are set to detonate in succession, at a period of time long enough so that all steps in the operation from the time the top wiping plug assembly is mounted in the lubricator until the completion of the stemming operation (and removal of the tubing, if desired) occurs before a time bomb detonates.

This explosive fracturing method provides positive control of the explosive during injection, keeps the time bomb, or any alternate ignition device, from having to be run on a wireline or dropped into the well at the start of operations, and has the important safety feature that a detonating or igniting device is not placed in the well until all of the explosive has been displaced below ground.

If the fissures or fractures 43 in the pay zone are at relatively shallow depths, and the volume of the liquid explosive to be employed is large, the method thus far described of injecting through a tubing string may result in some explosive being pumped into the fissures while the last of the liquid explosive is still being injected through the pipe string 23 at the surface. This is undesirable as experience has indicated that this may lead to a premature explosion which could propagate through the tubing up to the surface. Such a length of liquid explosive column can be avoided and our method practiced equally well by using the entire interior of the well (that is, inside the casing) for injecting liquid explosive. Such an arrangement is shown in FIG. 2.

Many of the items of equipment used here are common with those shown in FIG. 1. For example, the original assembly placed in the well, including the lower tubing 17 through the drillable packer 1-9 are the same as before, though there is no need to use an anchor tubing seal assembly 20. This packer assembly is placed in the well as described in connection with FIG. 1, and the packer 19 is set. However, in this case it may be desirable to set the packer so that the bottom of the lower tubing 17 is higher, for example two feet or more above the position shown in FIG. 3. This accommodates the special lower wiping plug assembly.

This special lower wiped plug assembly consists of a flexible casing wiper lower plug 50, the lips of which fit with wiping contact against the casing 14, but which are sufiiciently flexible so that plug 50 can be forced in collapsed position through the lower tubing 17, a short mandrel '51 of the order of a foot in length, and a lower plug 52 which will engage the inner Walls of tubing 17. Preferably below it is mounted a small metal cone 53.

The upper wiping plug assembly employs the same units shown in FIGS. 1 and 3 through 6, except that the top wiper plug 54 fits the inner surface of the casing 14. The lubricator 55 is identical in function and similarly fitted to lubricator 27, but again is the same diameter of the casing 14. The blowout preventer 21 and the explosive-proof valve 26 also are of a suitable size to fit the casing 14.

The lower wiping plug assembly is inserted into the casing 14 below the T and pipe 23, then the lubricator 25 installed, as previously indicated. As the lower wiping plug assembly is forced down by the liquid explosive, wiper plug 50 separates the explosive from the well fluids, as did the lower plug 39 in the arrangement shown in the other figures. Cone 53 finally enters the opening in packer 1'9 and the lower plug 52 immediately following bottom lip of flexible casing wiper lower plug 50, and the top of packer 19 forcing lower plug 52 through the packer and tubing 17. This force drags flexible casing wiper lower plug 50 behind plug 52, since they are attached. Accordingly, the lower wiper plug assembly ultimately passes through tubing 17 as did lower plug 39 in the other figures.

Valve 26 is opened after the last of the liquid explosive has been injected into the well and the upper plug assembly, including top wiper plug 54, is then forced down by the inert fluid, such as Water, injected through line 31. It is seen that this arrangement can be used in the same sequence as that previously described, the main difference being that in this case the same volume of liquid explosive occupies a much smaller length of column in the well. Placement of the stemming material 45 will customarily take place by running a string of tubing 16 into the well after the blanking plug 33 is seated in the stop 42 of the plug catcher or by simply dumping the stemming material in the casing at the top of the well through open valve 26.

If desired, the top of the lubricator may be supplied with a packing gland 56, through which a wireline 57 is run, carrying a sinker bar 58 (an ordinary cylindrical weight) at the lower end. The wireline 57 is run over a measuring sheave to a reel (not shown). Such devices have been used for many years to determine the location of an object in a well.

Such a wireline device can be used in either the arrangement shown in FIG. 2 or that shown in the other figures to determine that the top wiper plug has reached its bottom position at or adjacent the packer 19. If such a device is used, the sinker bar is simply lowered into the well at the time when it is expected that the blanking plug has entered the plug catcher, or if desired, at a time when the pressure gauge upstream of valve 36 registers an increase in pressure which can be interpreted as sealing off of the well at the location of the packer by the arrival of the upper wiping plug assembly. Measurement can thus provide assurance rapidly that the seal has been effected.

It should also be noted that ordinarily, quite effective stemming can be achieved simply by the column of water in the well above the seal, and, accordingly, the operator may decide not to add solids. \Ve prefer to use solids as at least part of the stemming materials in order that the effectiveness of the resultant explosion may be enhanced as far as opening up fractures into the pay formation away from the well is concerned.

We contemplate that in some cases there will only be one time bomb employed rather than two. In this case, the single time bomb used in this method should be attached by the soluble cord or rope to the upper wiping plug assembly, so that in all cases a detonator will rest at the bottom of the well after the explosive charge has been placed and before detonation.

Again, while we believe that it is desirable ordinarily to employ the blanking plug 33, it must be understood that this plug can be dispensed with. In this case, there is simply a lower confidence level in the sealing of the upper wiper plug assembly against the packer 19 at the conclusion of placing the liquid explosive.

It should also be understood that it is possible and somewhat time saving to force the upper wiper plug assembly down the well by pumping a batch of cement slurry into the region above it rather than water. This is particularly advantageous with the arrangement shown in FIG. 2, since in that case it is unnecessary to run tubing to conduct a stemming operation. After the desired volume of stemming material has been pumped into the well above the upper wiper plug assembly, one may follow this with water to complete the pumping operation.

It should also be stated that while the use of a clockwork mechanism or equivalent time ing apparatus for the detonators may be employed, we also comtemplate em- 8 ploying a detonator with an insulated connector cable which runs to the surface. This arrangement of apparatus is quite similar to that shown in FIG. 2, except that in this case instead of the conventional wireline 57, an insulated electric cable, for example, one of the slim wireline logging cables now available, is run through gland 56 and through the upper wiper plug 54, and blanking plug 33 down to the detonator. In this case after the detonator has reached its ultimate position, the operator can pass electricity through the conductor or conductors in the cable to initiate the detonation. The conductor must be permitted to pass relatively freely through the upper part of the upper wiper plug assembly in this case, since if such cable is attached to the lower detonator 36 it must be permitted by weight of that detonator plus any sinker bar attached to it, to fall the last few feet through the lower tubing 17 upon solution of the cord or rope 37.

We contemplate the use of a still different type of actuating mechanism. The detonator may contain no timing apparatus, but instead include a sensitive microphone with amplifier and a simple decoding mechanism. After the detonator has been placed in the well in its final position, the operator applies sonic energy to the casing or tubing (as the case may be) at the surface by a time-coded series of sonic energy bursts. This coded signal ultimately reaches the detonator and since its receiver is coded to handle this special signal, the sonic detonator will then explode.

We claim:

1. A method of explosively fracturing a subsurface formation adjacent a well, in which a liquid explosive is injected into said well and part into crevices or fissures in said formation, said injection taking place between lower and upper plugs isolating said explosive from well fluids until said lower plug is near the bottom of said Well and being effected by pumping an inert fluid into said well above said upper plug, the improvement comprising (a) stopping said injection of said explosive by sealing across said well at a predetermined level therein, whereby a volume of said explosive is still in the lower part of said well,

(b) attaching a timed detonator to said upper plug by a flexible member soluble in time in said explosive, the transverse dimensions of said member being suflicient so that the time of solution of said member is at least as great as the time for injecting said explosive into and moving it through said well.

(0) allowing the flexible member to dissolve, whereby the detonator previously attached to said upper plug becomes detached, so that said detonator may reach the bottom of said Well, and

(d) actuating said detonator.

2. A method in accordance with claim 1 in which the major component of said explosive is a nitroparaflin and said member is selected from the group consisting of cellulose acetate, cellulose triacetate, cellulose acetobutyrate and polyvinyl chloride.

3. A method in accordance with claim 1 including the steps of (a) attaching said member to a blanking plug 'affixed below and to said upper plug, said blanking plug being used for sealing said well, and

(b) maintaining said detonator, blanking plug, and upper plug isolated from contact with said explosive until after all said explosive has been injected into said well.

4. A method in accordance with claim 3 including the step of attaching a second detonator below and to said blanking plug by means insoluble in said explosive.

5. A method in accordance with claim 1 including the step of stemming said explosive prior to detonation by filling at least part of the well above said predetermined level by a nonexplosive material of at least the density of water.

References Cited UNITED STATES PATENTS 3,075,463 1/1963 Gilers et a1. 10221 3,002,454 10/ 1961 Chesnut 102-21 BENJAMIN A. BORCHELT, Primary Examiner J. V. DORAMUS, Assistant Examiner US. Cl. X.R.

166-308; l81.5 X C, .5 BH 

